This invention relates generally to a glow discharge etching process and more specifically to the etching of chromium by a glow discharge in an ambient atmosphere containing chlorine, oxygen or a noble gas, and water.
The etching of materials in a gas plasma formed from reactive gases is known. These processes can generally be divided into two types, termed "plasma etching" and "reactive ion etching". In plasma etching, the workpiece is placed in a chamber and a glow discharge is maintained by supplying RF energy to an electrode or electrodes which are external to the chamber. Typical power levels are 100 to 400 watts at 13.5 MHz and pressures are 0.05 to a few Torr. An example of this apparatus and process is shown in U.S. Pat. No. 3,951,709. In reactive ion etching, the workpiece is placed on an electrode in a chamber and a glow discharge is maintained by supplying RF energy to the electrode. Typical power levels are 0.1 to 0.5 watts/cm.sup.2 at 13.5 MHz and pressures are 5 to 50 milli-torr. The reactive ion etching process provides directional etching. An example of this process and apparatus is shown, for example, in U.S. Pat. No. 3,994,793. U.S. Pat. No. 3,951,709 discloses a process for forming semiconductor photomasks by etching metal layers, such as chromium or gold, on glass by a plasma etching process using gas mixtures of chlorine compounds such as CCl.sub.4 and C.sub.2 HCl.sub.3 mixed with oxygen. U.S. Pat. No. 3,994,793 discloses a process for selectively etching aluminum and aluminum alloys by a reactive ion etching process using a halogen containing gas mixture such as CCl.sub.4 and argon.
In forming conductive metallurgy patterns for integrated circuits of, for example, aluminum, problems of poor adhesion of the metallurgy to dielectric layers and of contamination of the semiconductor by alloying with the conductor metallurgy can occur. Therefore, auxiliary barrier layers of metals such as chromium, titanium, tungsten, and tantalum are used. These metal layers must be etched without significant attack on either the aluminum metallurgy, the silicon dioxide and nitride dielectric layers or the silicon substrate. Chlorine containing gases are known to rapidly attack silicon, for example, see U.S. Pat. No. 3,615,956, column 5, line 24-26.
Copending application Ser. No. 866,087, filed Dec. 30, 1977, now abandoned, entitled "Reactive Ion Etching Process for Metals" discloses a process employing gas mixtures of 5 to 20% by volumne of organic chlorine compounds, such as CCl.sub.4 or C.sub.2 HCl.sub.3 and 80-95% by volume of oxygen, which mixtures do not significantly etch aluminum or silicon while reactively ion etching the barrier metal layers.
In the process of application Ser. No. 866,087 SiO.sub.2 still measurably etches and, CCl.sub.4 contents of above about 10%, Si.sub.3 N.sub.4 etches. We have now found a process whereby chromium can be etched while SiO.sub.2 and Si.sub.3 N.sub.4 as well as silicon are not measurably attacked. Also, at higher CCl.sub.4 contents the etch rates of silicon containing materials are decreased substantially.
Harshbarger et al. "A Study of the Optical Emission from a RF Plasma During Semiconductor Etching", Kodak Microelectronics Seminar 1976, have reported that the addition of water to a plasma of CF.sub.4 and O.sub.2 impedes the etching of silicon and causes the formation of a residue. The effects of water being present are termed deleterious.